JavaScript PatternsLoved it
Every JS developer should read this book. It's a fantastic review of types, patterns and best practices. Personally, I prefer the NPM Coding Standards for my server side JS, but this book is still essential.

CSS3 For Web Designers [e-book]Loved it
Read the entire thing on my iPhone. Great, short read. Extremely useful.Totally sold me on rgba().

Mobile First [e-book]
Decent essay on why mobile is becoming more and more important. Best given to someone who's not already steeped in mobile.

JavaScript Web Applications[e-book] Loved it
The best book on web app fundamentals (w/backbone.js, spine, etc.) Covers MVC, HTML5 drag & drop and other APIs. Read it mostly at the gym and on airplanes.

Big Data Glossary [e-book]
Would be more useful as a PDF or even better a wiki.Secrets of the JavaScript Ninja [e-book]
Lots of crazy stuff about prototypes, testing, etc. Been in progress since like 2009 and it's not finished, so we get updates every couple of weeks.